The overall objectives of the proposed work are to exploit the process of skeletal myogenesis and the techniques of somatic cell hybridization in an effort to explore the control of phenotypic expression during cell differentiation. The cells to be utilized include chicken embryo and rat fetus myoblasts, myogenic cell lines of rat origin and a cell line derived from a murine rhabdomyosarcoma. Cell hybrids, cybrids, and reconstituted cells will be produced by polyethylene glycol-mediated fusion, and the fusion products identified both by selective and morphological markers. Myogenic capabilities will be assessed by means of such criteria as multinucleated muscle fiber formation, increases in enzyme activities characteristic of skeletal muscle, and by immunofluorescent staining with antibodies directed against skeletal muscle proteins.